In recent high-speed optical fiber digital communication systems, the Mach-Zehnder optical modulator is typically used to modulate an optical intensity in a transmitter. With reference to FIG. 1, the operation of the Mach-Zehnder optical modulator will be explained below. Incident light into an incidence end 8 of the modulator is equally divided into two by a Y-branch element 1. The divided lights are passed through the first and second optical waveguides 2, 3, respectively and are thereafter coupled at an optical coupling element 6. Here, over the first and second optical waveguides 2, 3, the first and second electrodes 4, 5 are provided, respectively. Depending on a signal voltage applied to the electrodes, refractive indexes of the optical waveguides thereunder can be independently changed to produce a phase difference between the two lights reaching the optical coupling element 6.
If the phase difference is 0, as shown in FIG. 2A, the first light 31 passing through the first optical waveguide 2 has the same phase as the second light passing through the second optical waveguide 3 at the optical coupling element 6 to give output light 33 with the biggest power. This corresponds to a light-ON state. On the other hand, when the phase difference is 180.degree., as shown in FIG. 2B, the first light has the reverse phase to the second light at the optical coupling element 6. Thereby the two lights interfere with each other to make the power of the output light 33 zero. This corresponds to a light-OFF state. A Mach-Zehnder modulator generally is composed of, as shown in FIG. 1, two optical waveguides with equal lengths. In this composition, if the same potential is given to two electrodes, the phase difference is 0 to give a light-ON state, and if the potential difference of half-wavelength voltage V.sub..pi. is given between them, the phase difference is 180.degree. to give a light-OFF state.
In optical communication, the power ratio of an ON level and an OFF level of modulated light, i.e., extinction ratio is an important factor. If the extinction ratio is small, the minimum input power necessary for a receiver to obtain a desired transmission quality must be greater, thereby shortening the transmission distance. In response to this, a modulator using lithium niobate(LiNbO.sub.3)(hereinafter referred to as `LN modulator`) which has a high extinction ratio more than 30 dB has been developed.
Also, in the optical communication, chirping that occurs in transmitted light is an important factor. The chirping is an optical frequency shift as shown in FIGS. 3A to 3C, which occurs simultaneously when optical intensity is modulated. It is called a positive chirping in the case that, as shown in FIG. 3A, a positive frequency shift occurs when turning light-OFF to light-ON and a negative frequency shift occurs when turning light-ON to light-OFF. On the contrary, it is called a negative chirping in the case that, as shown in FIG. 3B, the reverse frequency shift occurs. In FIGS. 3A and 3B, 41 and 43 indicate output optical waveforms and 42 and 44 indicate the chirping of modulated light. When a proper chirping occurs in transmitted light, optical pulses can be compressed to be transmitted with keeping the shape over a long distance. Utilizing this, when light with a wavelength of 1.5 .mu.m is transmitted using a fiber having zero dispersion in 1.3 .mu.m band, the transmission distance can be lengthened by the negative chirping occurred in the transmitted light. The Mach-Zehnder modulator is a device in which the amount of chirping can be controlled by the amplitude ratio of signal voltages applied to two optical waveguides.
In general, LiNbO.sub.3 has been used as the material for making the Mach-Zehnder modulator. Recently, a modulator made of semiconductor materials such as indium phosphide(InP) has been developed. When the semiconductor materials are employed, its phase variation per unit length of optical waveguide can be greater than that of LiNbO.sub.3, thereby reducing the applied voltage and the device size.
In the semiconductor Mach-Zehnder modulator, only reverse bias can be applied to optical waveguides to change the optical phase. This is because forward bias causes the emission of device. Thus, the voltage waveforms applied to two electrodes when modulated are as waveforms 51(applied voltage to the first electrode) and 52(applied voltage to the second electrode) shown in FIG. 4A. Here, the chirping of modulated light can be controlled by the amplitude ratio of the two signals.
On the other hand, in the semiconductor Mach-Zehnder optical modulator, the amount of light absorbed in an optical waveguide varies depending on the voltage applied to electrodes. Namely, loss at the optical waveguide varies depending on the applied voltage. Hereinafter, it is referred to as `loss variation`. FIG. 6 shows the loss variation to applied voltage at an optical waveguide. As shown in FIG. 6, the loss variation increases as the absolute value of the applied voltage is increased. The loss variation affects negatively to the extinction ratio and chirping of modulated light.
FIGS. 5A and 5B show optical output waveforms when light is modulated by the signals as shown in FIGS. 4B and 4C in a semiconductor optical modulator. In FIGS. 5A and 5B, 81 and 91 indicate the output light waveforms and 82 and 92 indicate the chirping of modulated light. When the modulation is conducted by the signal in FIG. 4B to occur the positive chirping, in the light-OFF state, voltages applied to the first and second electrodes are 0, -V.sub..pi., respectively. In this case, due to the loss variation, losses at the first and second optical waveguides are different. Thus, as shown in FIG. 2C, the intensities of the light 31 output from the first optical waveguide 2 and the light 32 output from the second optical waveguide 3 are not equal at the coupling element 6. Therefore, the two lights are not completely interfered when demultiplexed. As a result, some light will be emitted even in the light-OFF state, therefore decreasing the extinction ratio compared with the case of having no loss variation.
On the other hand, when the modulation is conducted by the signal in FIG. 4C to occur the negative chirping, in the light-ON state, voltages -V.sub..pi. are applied to the first and second electrodes. In this case, losses at both optical waveguides are occurred, thereby decreasing the output light as shown by the output waveform 91 in FIG. 5B. Thus, the extinction ratio will be further reduced as compared with the above case.
As explained above, when the loss variation exists, the chirping property must be deteriorated. When a Mach-Zehnder modulator has some loss variation, the output electric field E.sub.out of the modulator is represented as below: ##EQU1## EQU .PHI.=tan.sup.-1 [exp(.DELTA..alpha.)sin.DELTA..PHI./{1+exp(.DELTA..alpha.)cos.DELTA..PHI.} ], EQU .DELTA..alpha.=.alpha.(V.sub.2)-.alpha.(V.sub.1), .DELTA..PHI.=.PHI.(V.sub.2)-.PHI.(V.sub.1)
wherein Eo represents an incidence electric field amplitude into the modulator, .omega. represents an angular frequency, V.sub.1,V.sub.2 represent voltages applied to the first and second electrodes, respectively, .PHI.(V) represents a phase variation in the optical waveguide when a voltage V is applied, and .alpha.(V) represents a loss variation when a voltage V is applied.
The chirping of modulated waveform is represented as next: EQU df=d[.PHI.(V.sub.1)+tan.sup.-1 &lt;exp(.DELTA..alpha.)sin.DELTA..PHI./{1+exp(.DELTA..alpha.)cos.DELTA..PHI.} &gt;]/dt . . . (2)
In the second item within the square brackets of the equation (2), an excessive chirping due to loss variation is included. Herein, the chirping 82, 92 shown in FIGS. 5A and 5B are occurred in the case of having the loss variation of 2 dB in V=-V.sub..pi..
In FIGS. 5A and 5B, it is understood that the chirping due to loss variation is a negative steep chirping that appears before and after the light-ON. This chirping causes complex changes of the transmission characteristics in an optical fiber. Therefore, when such a modulator, as it is, is applied to a system which is designed for LN modulator having no loss variation, an optimum state thereof cannot be obtained, i.e., the system will require re-designing. Namely, due to the loss variation, the matching with LN modulators now available may be affected.
Furthermore, in the conventional semiconductor optical modulator, the average power of output light may vary depending on the way of applying voltage to each electrode. Herein, the attenuation ratio of an average power of output light compared to that in the case of having no loss variation will be referred to as `excess loss`. FIG. 7 shows excess losses to the voltage amplitude ratio r shown in FIG. 4A when a mark ratio which means a rate of light-ON occurrence in all signals is 1/2. According as r increases, the excess loss is increased. Due to this, when adjusting the modulator drive, the variation in output light level may cause the variation in system characteristics to make the adjustment difficult. In particular, in the case of amplifying the modulated light by an optical amplifier, the signal-to-noise ratio of output light may be deteriorated due to the reduction of input light level.
Japanese patent application laid-open No.5-72575 discloses an optical switch, which is not an optical modulator but belongs to Mach-Zehnder-type interference devices, that has a structure to reduce the optical crosstalk.
However, in the optical switch of Japanese patent application No.5-72575, loss variation, with which the present invention concerns, is not considered. Accordingly, it does not teach any solutions to the deterioration of chirping and the excess loss.